USING SDR TO EMBED WIMAX CHANNELS WITHIN THE TETRA FRAMEWORK Ling Gao, Ronan Farrell (Centre for Telecommunications Value Chain Research, National University of Ireland, Maynooth, Co. Kildare, Ireland;
[email protected],
[email protected])
ABSTRACT In recent years there has been a move to increase the data capabilities of TErrestrial Trunked Radio (TETRA) so as to provide secure broadband data capabilities. An initial enhancement (TETRA Enhanced Data Service, TEDS) has been agreed but there is a need to access additional spectrum. An investigation was carried out by European Telecommunications Standards Institute (ETSI) has concluded that a single standardised frequency band cannot be agreed and the concept of a tuning range for enhanced TETRA services can be deployed is gaining acceptance. In addition to needing flexibility in frequency, the enhanced TETRA services allows for more flexibility in the communication modes used so as to provide adaptability in applications. We propose that it is possible to deploy Software Defined Radio (SDR) technologies into the basestation to economically provide this level of flexibility, and to further extend the capability of TETRA services by deploying a WiMAX channel into the proposed TETRA tuning range and deliver true broadband data service while simultaneously support the original and enhanced TETRA services. 1. INTRODUCTION TETRA is a Private Mobile Radio (PMR) standard that has been developed by the European Telecommunications Standards Institute (ETSI) for the needs of the transport, civil and emergency services [1]. TETRA, first deployed in 1997, offers capabilities equivalent to the second generation of mobile phones with voice and limited data capabilities. TETRAPOL is another PMR standard, as developed by Matra Nortel Communications. TETRA and TETRAPOL are competitors in the market of PMR in Europe, in this paper we focus on TETRA services due to it is a more recent standard than TETRAPOL. We will compare the radio characteristics between TETRA and TETRAPOL later in this paper (Table 1). There is increased interest in delivery broadband data services over the TETRA network, for example video imagery of accident scenes. An enhanced form of TETRA (TEDS) has been agreed which can offer data rates of up to
600 kbps [2]. However successful deployment of TEDS requires additional spectrum to be allocation and this has proved to be problematic. In addition to the difficulty in agreeing a standardised spectrum allocation, enhanced TETRA supports a range of communication modes depending on individual user bandwidth and signal quality. This implies a greater complexity on the radio systems. Though the new TETRA services will offer improved capabilities, it is necessary to provide backward compatibility with existing TETRA users in the over 1000 currently deployed networks around the world [3]. The greatest challenges will be experienced by the TETRA basestations which must support new and legacy systems. SDR and specifically in the concept of flexible hardware transceiver systems, offers an economical solution to both the challenges of implementing TEDS and supporting legacy systems, and provides a development route for new TETRA services. In this paper we will provide an overview of the spectrum requirements for enhanced TETRA services and show that a WiMAX sub-channel can be integrated into the TETRA framework for true broadband services on demand. The paper will then present the requirements for a reconfigurable SDR platform with an investigation of various radio architectures to support the proposed and legacy schemes; plus the design challenges for this experimental platform. 2. SPECTRUM AND MODULATION REQUIREMENTS TETRA services were initially deployed in Europe in a 20 MHz band between 380 and 400 MHz as two 5 MHz bands with a 10 MHz duplex separation [1]. The majority of deployed networks operate in this band though other frequency ranges have been used for deployments in Asia and South America. To deploy the new enhanced TETRA data services additional spectrum is required to complement the existing band. An investigation by the European Telecommunications Standards Institute (ETSI) has concluded that a single standardised frequency band cannot be agreed. The Electronic Communications Committee (ECC) within European Conference of Postal and
Telecommunications Administrations (CEPT) has proposed a “tuning range” within which enhanced TETRA services can be deployed [4]. It recommends three bands within that tuning range, including the original TETRA band, as shown below (Figure 1). The tuning range requirements are further complicated as non-European deployments have used other frequencies ranges. One particularly interesting aspect is the Federal Communications Commission (FCC) proposed national public service network at 758-793 MHz [5] which would be attractive to any future TETRA-type network.
Figure 1 system tuning range
Within each 5 MHz slot, the new standard allows us to migrate from the existing 25 kHz TDMA π/4 DQPSK modulated channels to a range of other modulation schemes, (such as multi-level QAM and multi-carrier schemes, for example OFDM) and a range of signal bandwidths (25, 50, 100, 150 kHz). From a basestation perspective, these new modes offer a number of challenges, specifically maintaining noise and linearity performance over such a range of frequencies and handling the different modes of operation. For interoperability, it is important that basestations are capable of handling any client operating in the different modes, however constructing an RF transceiver with a frequency tuning range in excess of 20% (90 MHz centred on 425 MHz) is difficulty to achieve while retaining linearity and noise performance. If the upper FCC band was to be included, the tuning range would then exceed 50%, increasing the technical challenges further. The existing 25 kHz channels allowed for a highly optimised narrowband design philosophy. The new higher-efficiency schemes are flexible in terms of their bandwidth and receiver sensitivity. To address this issue of varying bandwidths, an optimal solution would be to capture the full signal bandwidth and then undertake signal channelisation and processing in the software or digital domains, following the SDR philosophy. This would place increased emphasis on the linearity, noise performance, and adjacent channel rejection of the RF stages, but would minimise cost and offer increased flexibility. 3. COMBINING WIMAX AND TETRA Enhanced TETRA allows for channel widths up to 150 kHz, offering users a range of data rates, up to 600 kbps. This is a significant improvement on existing TETRA services, however it does not offer data rates that would support full multimedia transmissions or rapid delivery of large files.
Though TEDS has identified a maximum channel width of 150 kHz, there is nothing inherent in the TETRA framework that prevents wider channels to be used. We propose that WiMAX (IEEE 802.16e) offers features that are highly suited to TETRA-type applications such as quality-ofservice guarantees and scalable OFDM access. The WiMAX standards allows for 1.25 MHz channel [6] which would allow up to three 1.25 MHz WiMAX channel to be deployed with the remaining spectrum then used to support voice and data services whether using TETRA or TEDS, thus maintaining legacy support (Figure 2). The key advantage to using the WiMAX standard is scalable OFDM access schemes (OFDMA) where users are dynamically allocated bandwidth as needed for their application, according to their quality of service metric and allow users to obtain bursts of data throughput of up to 6 Mbps when needed. WiMAX presents low cost of delivery of higher data rates over large geographical areas and also perform very well in mobile conditions. With WiMAX’s enhanced channel efficiency of up to 5 bits/hertz, creator number of users plus applications can be supplied. The use of high data rate OFDMA modulations brings into challenging requirements for the transmitter in terms of spectral quality and Error Vector Magnitude (EVM). Also the receiver faces some difficulties. The high EVM required is difficult to attain because it demands a high Signal-toNoise Ratio (SNR) from the Low Noise Amplifier (LNA), about 35 dB. Other challenges are that the receiver must exhibit low power consumption, high bandwidth and high dynamic range. [7]
Figure 2 5 MHz TETRA channel
If basestations are to be designed using full channel capture and channelisation in the digital domain, implementing this WiMAX sub-channel requires only a small modification of the software implementation of the physical layer and then subsequently a separate WiMAX stack. 4. SOFTWARE DEFINED RADIO PLATFORM REQUIREMENTS TEDS requires an infrastructure that can support a tuning range and a wide range of modulation schemes of varying bandwidth. In addition to the identified TEDS frequencies, a range of other frequencies are also likely to be needed to be supported, such as 700-800 MHz. As volumes of TETRA basestations are relatively small, a common platform would be of economic benefit and also allow for improved
interoperability of mobile devices. This combination makes a compelling case for use of a software defined radio approach. As TEDS uses similar modulation schemes though at different operating frequencies and channel bandwidths, the challenges for a SDR platform are focused on the RF stages rather than the software framework. Specifically there are demanding receiver requirements on signal sensitivity, adjacent channel rejection, and linearity. These issues were manageable when dealing with narrowband signals at a specific frequency but become much more challenging when dealing with a wide tuning range. One particular issue is the problem of the transceiver filter which must be wideband or reconfigurable in some way. This will limit our ability to minimize adjacent channel interference. To address the issue of varying sub-channel widths, it will be necessary to undertake full channel capture and subsequently digitally undertake channelisation, filtering and de-modulation. If this approach is taken minimizing wideband noise contributions from the electronics and adjacent channels becomes particularly important. To develop a new system suits our proposal, the main radio characteristics of the TETRA, TEDS, TETRAPOL and WiMAX standards are studied as follow:
Frequency (MHz) Spectrum Allocation Duplex Spacing (MHz) Channel BW (kHz) Channel Spacing (kHz) Access Scheme Modulation
Tx Power (dBm) Rx Sensitivity (dBm) Efficiency (bits/Hz)
TETRA
TEDS
TETRA POL 80/380/ 450 similar to TETRA similar to TETRA
